Ar+ ion irradiation-induced reorganization of colloidal silica nanoparticles in Langmuir-Blodgett monolayers

The Ar+ ion irradiation-induced reorganization of Langmuir-Blodgett monolayers of colloidal silica nanoparticles with a diameter of 220 nm and 450 nm deposited on silicon substrate is investigated in a wide range of ion fluences. The cluster formation, average cluster size, ion beam-induced charging and discharging of particles, heating effects, the ion beam-induced viscous flow as well as the swelling of the Si substrate as a function of the ion fluence are discussed. Inhomogeneous particle distribution causes strong nonlinear Si swelling inside wide defect-channels between the clusters and linear Si swelling inside narrow channels between the particles. At low ion fluences particle charging and Coulomb repulsion forces govern the particle reorganization process, while at high ion fluences ion beam-induced viscous flow and hydrodynamic forces between the particles and the underlying substrate play major role. Two different types of patterns: smaller compact clusters of few particles and longer chain-like clusters of dozens of particles are observed. The dynamics of the reorganization process in both cases is treated analogous to the dynamics of colloidal suspensions based on the Tanaka-Araki model. The results of the model are comparable to the experimentally observed cluster characteristics.